Power transmitting devices known in the prior art are naturally divided into the following four categories: friction drives, hydraulic (hydrostatic or hydrodynamic) transmissions, electric transmissions, and cogwheels or gears. All these drives have important disadvantages. Thus friction drives that depend upon friction force acting between moving parts in contact to transmit motion have very limited transmission capacity and therefore are usually bulky and heavy, and suffer from slippage between cooperating members which diminishes their durability and efficiency, and renders their reliability questionable. Hydraulic transmissions are capable of coping with large loads while retaining compact size, but are complicated and expensive, and feature poor efficiency and not always satisfactory reliability. Electric transmissions are complicated, heavy and expensive, and definitely not suitable for application in high power drive lines of aerial vehicles. Gears are far superior to all aforementioned power transmitting devices in terms of transmission capacity/weight, efficiency and reliability, and are used as the only power transmitting devices in aviation drives (with exception of those of very small power). However also gears have some disadvantages: Cooperating gear teeth slide against each other (rarely used Wildhaber-Novikov's meshing being an exception), which diminishes gear's transmission capacity and reliability, and causes gears require good lubrication, as lack of lubrication leads to seizure and catastrophic failure. Recent trends in large bypass ratio turbofan engines technology (large diameter fans driven by turbine via a gear) impose particularly high demands on the gears as far as reliability, safety and transmission capacity are concerned.
Gears, in which teeth on the pinion are replaced by rollers, as described e.g. in U.S. Pat. No. 5,247,847 haven't earned wider acceptance, and rollers that cyclically engage and disengage spur gear teeth seems to be the main reason of this state of affairs.
Thus there is a need for a compact, lightweight, extremely reliable gear, insusceptible to seizure, capable of meeting stringent requirements of aviation industry.